Construction Of Z-scheme Fe-based Photoanode And Study On Photoelectrochemical Water Oxidation Performance

To find green technologies as sustainable ways to solve the energy crisis and improve the quality of the environment is very important.In recent years,hydrogen energy is entering thousands of households as a clean energy,and the use of inexhaustible solar energy is considered to be an effective strategy to prepare hydrogen energy.In the process of water splitting,the oxygen-generating half-reaction is a four-electron process with a Gibbs free energy greater than 0 and becomes the rate-controlling step of the entire photocatalytic water splitting reaction.Therefore,the preparation of high efficiency photoanode material is the key to improve the efficiency of photocatalytic water splitting.Iron-based photocatalysts have become a research hotspot in the field of photocatalysis due to their abundant earth content,environmental friendliness,and narrow band gaps.However,its application in photocatalysis is severely restricted by the serious?-Fe₂O₃ bulk recombination of indirect band gap semiconductor and the short lifetime of photocarrier.The Z-scheme photocatalytic system,which simulates the natural photosynthesis,can not only promote the rapid charge separation and transfer,but also retain strong REDOX sites,thus becoming an effective strategy to improve the performance of iron-based photocatalysis.In recent years,many researchers have constructed a series of Z-scheme photocatalysts to effectively improve the photocatalytic performance.However,Current technical means cannot give the direct evidence of Z-scheme migration mechanism,which is a prerequisite for the design and preparation of efficient Z-scheme photocatalysts.Therefore,based on the current situation of Z-scheme photocatalytic system,we put forward the idea and research focus of this paper.The direction of self-built electric field determines the migration direction of the photoinduced charge.Firstly,the two-dimensional ordered iron-based photocatalysts are applied to explore the directional transfer of charges at the interface,and clarify the Z-scheme migration mechanism of catalyst.secondly,the intensity of interfacial electric field determines the separation efficiency of photogenerated charge,and the driving force of charge directional transfer is controlled by tuning the Fermi level of the two materials,correspondingly the behaviors of photoinduced charge to fully proves the Z-scheme transfer mechanism.Finally,the photocatalytic water oxidation performance of iron-based photoanode material was improved.In this work,we selected ordered CdS/Ti-Fe₂O₃,Ti-Fe₂O₃/In₂O₃ photocatalytic materials as the models to investigate the Z-scheme migration mechanism on the photoinduced charge by surface photovoltage and photoelectrochemical methods.And the Z-scheme migration mechanism was fully proved by the regulation of the interfacial electric field,which provided ideas and means for designing more efficient Z-scheme photoanode materials.The specific content is as follows:(1)We constructed CdS/Ti-Fe₂O₃ photoanode as a model to establish a surfacephotovoltaic test method,which was used to clarify the Z-scheme migration mechanism of the photocatalytic system.The surface work function results show that the driving force at the interfacial electric field between Ti-Fe₂O₃ and CdS indicates that charges follow the Z-scheme path.Electrons were detected in CdS side and holes were detected in Ti-Fe₂O₃ side through surface photovoltaic technology,which proved the Z-scheme migration mechanism of CdS/Ti-Fe₂O₃.Because Z-scheme mechanism is a two-photon process,we adopted a double-beam strategy to deeply explore the charge behavior at the interface,and fully verified the Z-scheme mechanism of the composite photoanode.A series of photoelectrochemical tests showed that the CdS/Ti-Fe₂O₃ effectively enhanced the water oxidation performance of Ti-Fe₂O₃.(2)Z-scheme transport is a process of directional charge transport and migration at the interface.In order to ensure smoother interface charge transport and further improve the photocatalytic performance of iron-based materials,we choose ZnFe₂O₄(ZFO)with a high degree of compatibility with?-Fe₂O₃ to construct the Ti-ZFO/Ti-Fe₂O₃ photoanode with a smooth interface.The Ti-ZFO/Ti-Fe₂O₃ exhibits excellent photocatalytic performance,with a photocurrent of 2.16 m A/cm² at 1.23 V vs.RHE,which is 3 times higher than that of pure Ti-Fe₂O₃,indicating that the fast interfacial charge transport improves the water oxidation performance of Ti-Fe₂O₃.The surface photovoltage spectrum shows that more holes migrate to the Ti-Fe₂O₃ surface driven by the enhanced interfacial electric field,indicating that the electrons of Ti-Fe₂O₃recombine with the holes of Ti-ZFO at the interface,which fully proves its Z-scheme transport mechanism.(3)retaining strong oxidation sites is the characteristic of the Z-scheme migration mechanism.In order to further improve the photocatalytic water oxidation performance of Fe-based materials,we select In₂O₃ with positive valence band position to construct Ti-Fe₂O₃/In₂O₃ photoanode.Under the premise of rapid interfacial photogenerated carrier transport,the holes on positive oxidation potential participate in the water oxidation reaction.The photochemical test shows that Ti-Fe2O3/In2O3 photoanodes have good photocatalytic performance for water oxidation.In addition,we adjust the interface electric field intensity by regulating the Fermi level position of Ti-Fe₂O₃,and further explore the charge migration behavior driven through the interface electric field,which fully proves the Z-scheme migration mechanism of Ti-Fe₂O₃/In₂O₃.(4)In order to design a photocatalyst with strong redox ability,we used the Ti-ZnFe₂O₄/In₂O₃(Ti-ZFO/In₂O₃)Z-scheme photoanode as a model to research the relationship between reaction sites and photocatalytic water oxidation performance,so as to further optimize the photocatalytic system.Compared with?-Fe₂O₃ in Chapter 4,ZnFe₂O₄ has a more negative conduction band position and a narrower band gap,under the action of interfacial electric field,the charge can be rapidly transferred and separated,the photogenerated electrons and holes migrate to the strong reduction and oxidation sites to participate in the photocatalytic reaction,respectively.As expected,Ti-ZFO/In₂O₃ shows the higher photocurrent density of 2.2 m A/cm² at 1.23 V vs.RHE,which is 7.3 times higher compared with pure In₂O₃ and Ti-ZFO.